1. Field of the Invention
This invention generally relates to Fourier filters and wafer inspection systems. Certain embodiments relate to a one-dimensional Fourier filter configured to be included in a bright field inspection system such that the bright field inspection system can be used for broadband dark field inspection of a wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers. Defect inspection is currently being performed using techniques such as bright field (BF) imaging, dark field (DF) imaging, and scattering. The type of inspection tool that is used for inspecting semiconductor wafers may be selected based on, for example, characteristics of the defects of interest and characteristics of the wafers that will be inspected. For example, some inspection tools are designed to inspect unpatterned semiconductor wafers or patterned semiconductor wafers.
Inspection tools for unpatterned wafers are generally not capable of inspecting patterned wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens or another collector is directed to a single detector that generates a single output signal representative of all of the collected light. Therefore, light scattered from patterns or other features on the specimen will be combined with other scattered light. As such, light scattered from patterns or other features on the wafer can not be separated from other scattered light thereby hindering, if not preventing, defect detection.
Patterned wafer inspection is of particular interest and importance to the semiconductor industry because processed semiconductor wafers usually have a pattern of features formed thereon. Although inspection of unpatterned wafers, or “monitor wafers,” which have been run through a process tool, may be used as a gauge for the number and types of defects that may be found on patterned wafers, or “product wafers,” defects detected on monitor wafers do not always accurately reflect the defects that are detected on patterned wafers after the same process in the same process tool. Inspection of patterned wafers is, therefore, important to accurately detect defects that may have been formed on the wafer during, or as a result of, processing. Therefore, inspecting patterned wafers or product wafers may provide more accurate monitoring and control of processes and process tools than inspection of monitor wafers.
Many inspection tools have been developed for patterned wafer inspection. Some patterned wafer inspection tools utilize spatial filters to separate light scattered from patterned features from other scattered light such that the other scattered light may be separately detected. Since the light scattered from patterned features depends on various characteristics of the patterned features such as lateral dimension and period, the design of the spatial filter also depends on such characteristics of the patterned features. As a result, the spatial filter must be designed based on known or determined characteristics of the patterned features and must vary as different patterned features are being inspected.
One type of spatial filter that may be used as described above is a Fourier filter. Fourier filters are relatively useful for filtering light from repetitive patterns such as memory array formed on a wafer. However, Fourier filters can have adverse effects on the transmitted light because the filters are often in the form of periodic blocking bars. These type of filters can diffract light into undesirable directions thereby degrading the imaging quality, which is commonly referred to as ringing or side lobes. Most Fourier filters perform well for inspection systems that use coherent light such as that generated by a laser. However, for inspection systems that use broad band light, it is difficult to use a Fourier filter because the performance of Fourier filters is strongly dependent on wavelength. In addition, the Fourier filter can produce significant distortion at the image plane, which adversely affects the ability of the inspection system to detect defects on the wafer with high accuracy.
Conventional dark field imaging generally uses a circular symmetric illumination aperture with a complementary imaging aperture. While a circular aperture works well for filtering light reflected from wafer, a circular aperture is less than optimum for filtering the light diffracted from repetitive patterns. For example, for many one-dimensional patterns such as metal-1 (M1) repetitive line structures, a conventional dark field aperture does not block all of the diffraction orders completely due to its circular shape. Therefore, such an aperture cannot completely eliminate the pattern and background noise thereby limiting the achievable sensitivity of the system for defect detection on wafers containing one-dimensional or highly asymmetric patterns.
Accordingly, it would be advantageous to develop a Fourier filter that can be used in a bright field inspection system such that the bright field inspection system can be used for broadband dark field inspection of a wafer containing one-dimensional or highly asymmetric patterns and that can substantially eliminate pattern and background noise without producing distortions at the image plane of the system such as ringing.